1. Field of the Invention
The present invention relates, generally, to a torque converter having a clutch assembly, and, more specifically, to a torque converter having a lock-up clutch assembly that includes a floating friction disk.
2. Description of the Related Art
In automotive applications, engine torque and speed are translated between a prime mover, such as an internal combustion engine, to one or more wheels through the transmission in accordance with the tractive power demand of the vehicle. Hydrokinetic devices, such as torque converters, are often employed between the internal combustion engine and its associated transmission for transferring kinetic energy therebetween.
As illustrated schematically in FIG. 1, torque converters 10 include impeller assemblies 12 operatively connected for rotation with the torque input from the internal combustion engine, a turbine assembly 14 fluidly connected in driven relationship with the impeller assembly and a stator or reactor assembly 16. These assemblies together form a substantially toroidal flow passage for kinetic fluid in the torque converter. Each assembly includes a plurality of blades or vanes that act to convert mechanical energy to hydrokinetic energy and back to mechanical energy. The stator assembly 16 of a conventional torque converter is locked against rotation in one direction but is free to spin about an axis in the direction of rotation of the impeller assembly 12 and turbine assembly 14. When the stator assembly 16 is locked against rotation, the torque is multiplied by the torque converter. During torque multiplication, the output torque is greater than the input torque for the torque converter.
Conventional torque converters often employ clutches interposed between a torque input member and the turbine assembly which are engaged and “lock up” at high speed ratios (N2/N1) of between 0.88-0.98, where N1 is the rotational speed input to the converter and N2 is the rotational speed output from the converter. When the “lock-up” clutch is locked there is a direct torque translation between the torque input member and the transmission through the turbine assembly. Conventional lock-up clutches are generally engaged only after the torque converter has reached a predetermined speed. The locked lock-up clutch eliminates the slip inherent with the fluid coupling and results in an efficiency gain for the torque converter. Depending on throttle position and shift strategy, this may occur at vehicle speeds of between 40 and 45 mph.
As noted above, lock-up clutches are typically located between the turbine assembly and the front torque converter member or cover. The lock-up clutch includes an annular piston or disc that is rotatably supported on the turbine assembly. A frictional material or facing element is bonded to either the outer face of the annual piston or the front cover member. Thus, the frictional material is bonded to one member in angular sliding contact with the other. The frictional material is specifically chosen with a particular co-efficient of fiction to provide lock-up at a predetermined torque load. The torque load at which the lock-up clutch locks may also be influenced by the formation of circuitous grooves cut into the frictional material that allows a flow of hydraulic fluid along the grooves for cooling as indicated in U.S. Pat. Nos. 5,566,802 and 6,000,510. Both of these patents are assigned to the assignee of the present invention and their disclosures are incorporated herein by reference.
Directly applying the friction material to the front cover member of the torque converter has been known to present certain assembly and operational drawbacks. Bonding a ring of delicate friction material having grooved recesses on the inner face of the cover member without damage or misalignment is problematic. Placing friction material in the cover member also requires additional machining steps to be performed on the interior of the cover member to establish the bonding surface. Conversely, applying the friction material to the annular piston has certain drawbacks as well. Specifically, torque converters are often constructed in a series of steps that may occur at different physical locations and during different points in the overall construction of the transmission. If the friction material is bonded to the annular piston of the torque converter, this must be done somewhat early in the transmission assembly process. This exposes the delicate frictional material to damage throughout the remainder of the assembly of the transmission that may possibly cause early failure of the torque converter. Even if the frictional material does not suffer significant damage, small impacts or nicks can change the design performance of the lock-up clutch causing operational inconsistencies.
In an attempt to overcome these assembly drawbacks, it is known to provide a friction plate having frictional material bonded to it that is then welded into the torque converter assembly. This type of friction plate may be located between the annular piston and the input member or front torque converter cover as indicated in U.S. Pat. No. 5,878,860. The '860 patent is assigned to the assignee of the present invention and its disclosure is incorporated herein by reference. As noted above, the friction plate is welded to the annular piston of the torque converter typically late in the transmission assembly process so that its exposure to assembly damage is minimized.
While torque converters having a lock-up clutch assembly of the type generally known in the related art have preformed reasonably well for their intended purposes, some disadvantages remain. More specifically, torque converters remain complex devices that take numerous assembly steps and are costly and time consuming to produce. When a friction plate for the lock-up clutch is employed, it must still be welded to the annular piston about its outer edge in close proximity to the frictional material. This again subjects the friction material to possible damage and may cause undetected heat warping of the plate. Furthermore, the assembly of this type of fixed friction plate requires extremely accurate placement and alignment against the annular piston prior to and during the welding procedure. Any misalignment will cause improper operation of the lock-up clutch and uneven wear in the friction material since the welded and fixed friction plate will not be able to account for the misalignment. Finally, the friction plates of conventional torque converter lock-up clutches are full disc-shaped plates that are operatively supported at their inner diameter by a shaft or bushing of the torque converter. A full size friction plate is employed to help counter any distortions that may be imparted to the friction plate during its welding to the annular piston. However, employing a full size friction plate adds additional rotating mass and inertia within the torque converter adding parasitic drain, thereby decreasing torque converter efficiency and adding to operational costs.
Thus, there remains a need in the art for a torque converter having a lock-up clutch assembly that employs a friction plate that may be easily constructed in a separate process to protect the bonded friction material, and that may be easily inserted into and retained within the torque converter during assembly. In addition, there remains a need in the art for such a torque converter that employs a friction plate that is not fixed to the annular piston and is capable of accounting for wear and misalignment within the torque converter. Finally, there remains a need in the art for such a torque converter that has a friction plate that results in reduced weight and reduced parasitic losses and thus improves torque converter efficiency.